An ink jet system of discharging ink through ink-discharging openings such as nozzles is used in many printers because the system is compact, inexpensive and capable of forming an image on a printing medium without contacting therewith. In the ink jet system, a piezo ink jet system of discharging ink by utilizing deformation of a piezoelectric element or a thermal ink jet system of discharging ink by utilizing the boiling phenomenon of ink with thermal energy is characterized by excellent properties such as high resolution and high-speed printing.
At present, raising the speed and improving the image quality when printing on paper or a non-water-absorptive recording medium such as plastics are critical issues. This is because there are practical problems wherein particularly when the drying of liquid droplets takes time after printing, image bleeding occurs easily, and interference between discharged droplets occurs upon mixing of adjacent ink droplets, thus preventing formation of sharp images, and when a non-water-absorptive recording medium is used, the drying of a solvent is very slow, which may necessitate drying of printed materials just after printing without laying the printed materials one on top of another. The interference between discharged droplets refers to the phenomenon where adjacently discharged liquid droplets unite with each other because of the tendency of the adjacently discharged liquid droplets to reduce their surface energy (and to decrease their surface area). Upon unification of adjacently discharged liquid droplets, the liquid droplets shift from the position of impact on a recording medium so that particularly when a thin line is drawn with colorant-containing ink, the width of the line is made uneven, and when a solid image is drawn, the solid image is made uneven.
To suppress the bleeding of an image and the interference between discharged droplets, an ink for ink jetting which is fixed by curing not through evaporation of an ink solvent but with radiation is proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 5-214279) as one technique of accelerating the curing of ink. However, a pigment dispersion is used as a coloring component, and flocculation of the pigment causes clogging of nozzles to make stable discharge of ink difficult.
To form images excellent in transparency and hue without using any pigment, UV-curing ink using a dye as a colorant is also disclosed (see, for example, U.S. Pat. No. 4,303,924). This ink suffers from a problem of insufficient storage stability since undesirable polymerization reaction occurs easily during storage. Further, the ink contains electroconductive salts that are sometimes poor in solubility in ink, so there is concern about defective printing due to precipitation of the salts during long-term storage.
To satisfy both storage stability and high-speed drying characteristics, a technique of using an ink of two-liquid type to react the two on a recording medium is proposed; for example, there are disclosed a method which involves allowing a liquid having a basic polymer to adhere to a recording medium and then using an anionic dye-containing ink in recording (see, for example, JP-A No. 63-60783), a method that involves applying a cationic substance-containing liquid composition and then applying an anionic compound- and colorant-containing ink (see, for example, JP-A No. 8-174997), and a recording method of using a photo-setting resin in one liquid and a photopolymerization initiator-containing ink in the other liquid (see, for example, Japanese Patent No. 3478495).
However, these methods suppress the bleeding of an image by precipitation of a dye itself and are not capable of suppressing the interference between discharged droplets, and since an aqueous solvent is contained, the drying speed is slow, and further a precipitated dye can easily spread unevenly on a recording medium causing deterioration in image quality.